1. Field of the Invention
This invention concerns methods and apparatuses for analyzing and correcting medical imaging data of a subject.
2. Description of the Prior Art
In the medical imaging field, several nuclear medicine emission imaging schemes are known. For example PET (Positron Emission Tomography) is a method for imaging a subject in 3D using an ingested radio-active substance which is processed in the body, typically resulting in an image indicating one or more biological functions. FDG, for instance, is a glucose analog which is used as the radiopharmaceutical tracer in PET imaging to show a map of glucose metabolism. For cancer for example, FDG is particularly indicated as most tumors are hypermetabolic, which will appear as a high intensity signal in the PET image. For this reason, PET imaging is widely used to detect and stage a wide variety of cancers. The level of glucose activity is usually highly correlated with the aggressiveness and extent of the cancer, and, for example, a reduction in FDG signal between a baseline and a follow-up scan is often indicative of a positive response to therapy.
A key criterion used in evaluating suspicious lesions in a PET scan is the Standardized Uptake Value (SUV). This value is computed from the number of counts of emission events recorded per voxel in the image reconstructed from the event data captured in the PET scan (coincidence emission events along the line of response). Effectively the SUV's purpose is to provide a standardized measure of the spatial distribution of radiotracer concentration throughout the imaged portion of the body.
Conventionally, PET scans are acquired using a static protocol, producing a single image volume representing the average counts (per voxel) detected over a fixed period of time following a given interval between radiotracer injection and image acquisition.
The interval between radiotracer injection and PET acquisition is intended to allow the biological system to reach a steady state equilibrium, with respect to radiotracer distribution. However, with many clinical protocols using an interval of 45-60 mins for 18F-FDG, this equilibrium is often not achieved. As such, small differences in the timing of the acquisition window, post injection (PI) of radiotracer, can significantly affect the uptake (or SUV) measured for a malignant region.
Despite this fact, variations occur in the PI interval during the clinical imaging process, which may in turn result in the misinterpretation of a difference in uptake (or SUV) between two scans, when in fact it may purely be as a result of different PI intervals.
Currently, efforts are made to maintain consistency in PI interval in a clinical protocol, but variations do still occur as not all factors can be controlled. Despite these variations, typically no attempts to correct for the potential impact of such differences are made; furthermore, the clinician reading the scan is often unaware of such differences.
Some clinical applications that can simultaneously load multiple data volumes highlight to the user any significant differences in the PI interval between the two scans have been previously considered (for example, TrueD from Siemens Healthcare). However, this is only intended to inform the user that such a different exists and does not attempt to correct for the uptake itself.